Jonathan Blow

Luxuries in mainland tends to cost much more comparing to HK. The difference is so big that a whole bunch of professional parallel traders（水客） exists, whose daily job is to travel to HK buying loads of stuff then resell it to mainlanders at a higher but still attractive price

The reasons they tend to buy these things in HongKong rather than in mainland are just like Lux said the cost is higher in mainland I think higher tax rate may be matter. And the goodwill of hk shops are better than mainland, which means hk shops are not likely selling fake goods

Also, buy these luxury goods in Hong Kong could be a way of money laundering for some of them because they can be traced if they use the illegal/unrecoreded income in mainland.